The invention relates generally to lightwave communication systems and, more particularly, to coding techniques used in the transmission of information in dense wavelength division multiplexing systems.
Optical fiber is becoming a preferred transmission medium for many communication networks because of the speed and bandwidth advantages associated with optical transmission. In particular, wavelength division multiplexing (WDM) is being used to meet the increasing demands for more speed and bandwidth in optical transmission applications. In its simplest form, WDM is a technique whereby parallel data streams modulating light at different wavelengths are coupled simultaneously into the same optical fiber. A WDM signal is typically viewed as a composite signal comprising a parallel set of optical channels sharing a single transmission medium, wherein each optical channel uses a slightly different frequency (wavelength of light). Although each optical channel actually includes a range of frequencies (wavelengths), those skilled in the art typically refer to an optical channel in terms of its center wavelength. As such, the terms xe2x80x9coptical channelxe2x80x9d, xe2x80x9cwavelength channelxe2x80x9d, and wavelength are typically used interchangeably in the WDM context to refer to a constituent optical signal within the composite WDM signal.
With recent advances in optical networking technology, system manufacturers are now contemplating WDM systems that carry, for example, as many as 40, 80, or more optical channels in a single fiber and with bit rates up to 10 Gbps per channel. However, despite the many advantages of WDM, there are certain limitations that arise as a result of using a separate optical channel for carrying traffic from each source or user, which is a typical transport arrangement in WDM systems. For example, the number of users that can be supported by a WDM system is limited by the number of available optical channels. The total bandwidth in a WDM system may also be used inefficiently if optical channels are not being used to transport information at the maximum possible bit rate. Furthermore, the electronic components used for processing the optically transmitted information at nodes in a WDM system lag behind photonic components in terms of transmission speed capabilities. Although many user systems supply information in parallel format, prior WDM transport schemes incorporate multiple serial-to-parallel and parallel-to-serial conversions, thus resulting in latency and delay.
In optical transmission systems, it is also well-known that fiber nonlinearities can affect the integrity of data transmitted in the optical channels, thereby degrading system performance. Some of the nonlinearities include, for example, four wave mixing, stimulated Raman scattering, and stimulated Brillouin scattering, to name a few. For a more detailed description of these optical nonlinearities, see, e.g., Kaminow et al., xe2x80x9cOptical Fiber Telecommunicationsxe2x80x9d, Vol. IIIA, pp. 199-203, 212-225, and 239-248 (1997) and Maeda et al., xe2x80x9cThe Effect of Four-Wave Mixing in Fibers on Optical Frequency-Division Multiplexed Systemsxe2x80x9d, Journal of Lightwave Technology, Vol. 8, No. 9, pp. 1402-1408 (1990), each of which is incorporated by reference herein. Four wave mixing is of particular concern for WDM systems.
Briefly, four wave mixing is a third order nonlinearity in which three input signals generate a fourth signal which may degrade the system""s performance via crosstalk. More specifically, three waves of frequencies (e.g., fi, fj, and fk) interact through the third order electric susceptibility of the optical fiber to generate a fourth wave of frequency (ffwm) defined by ffwm=fi+fjxe2x88x92fk. Thus, three co-propagating waves give rise to nine new components. In a WDM system, this interaction occurs for every possible choice of three waves (i.e., optical channels), which can result in the generation of hundreds of new components. As such, four wave mixing contribution to crosstalk and the resultant signal to noise degradation can be especially problematic for WDM systems having a plurality of optical channels.
Four wave mixing effects also depend on the coherent presence of energy. As such, four wave mixing is a concern for intensity modulated systems, e.g., where the presence of a pulse indicates the symbol or bit xe2x80x9conexe2x80x9d and the absence of a pulse indicates a symbol or bit xe2x80x9czeroxe2x80x9d. More specifically, if a bit xe2x80x9conexe2x80x9d is being transmitted in one or more adjacent optical channels in the same segment of the optical fiber, then the effects of four wave mixing can be greater than a case where one channel has a bit xe2x80x9conexe2x80x9d and one or more of the other channels has a bit xe2x80x9czeroxe2x80x9d.
FIG. 1A illustrates this principle in a simplified example of a typical WDM transmission scheme in which data is transmitted serially in each of the optical channels so that data transmission in any given optical channel is independent of data transmission in another optical channel. More specifically, FIG. 1A shows three optical channels 101-103 (wavelengths xcex1, xcex2, and xcex3), each carrying serially transmitted data represented as bit sequences, e.g., bit sequence a0-a4 in optical channel 101, bit sequence b0-b4 in optical channel 102, and bit sequence c0-c4 in optical channel 103. For this example, bits a0, b0, and c0 each occur at time slot t0, bits a1, b1, and c1 at time slot t1, and so on. In this way, the bits in each of the individual optical channels at a particular time slot (e.g., a bit period) are considered to be coincidental bits, e.g, a0, b0, and c0 at time t0 shown in block 104. Because effects of four wave mixing depend on bit values, e.g., xe2x80x9conesxe2x80x9d or xe2x80x9czerosxe2x80x9d, of coincidental bits in different optical channels, traditional WDM transmission schemes that transmit data serially and independently in each optical channel therefore are particularly susceptible to crosstalk contributed by four wave mixing.
FIG. 1B illustrates how propagation delays in the individual optical channels can further complicate the identification coincidental bits having bit values which may give rise to four wave mixing effects. In particular, it is well known that light pulses may travel at different velocities in different optical channels of a WDM signal, i.e., at different wavelengths. For example, FIG. 1A shows how bits a4, b4, and c4 are coincidental at time slot t4 (e.g., within a bit period) at the transmitter end. However, these bits may not be coincidental (e.g., within a bit period) at the receiver end because of different delays experienced at different wavelengths. As shown in FIG. 1B after propagation delays, bit a4 is coincidental at time t4 with bits b3 and c2 as shown by block 105. So, bit values of coincidental bits are important both at the near-end (e.g., transmit end) and far-end (e.g., receive end) of a system since four wave mixing may develop as a result of propagation delays in the optical channels.
Consequently, advantages of using WDM in optical transmission are tempered by the problems caused by fiber nonlinearities, inefficient bandwidth utilization, and delays associated with serial-to-parallel/parallel-to-serial conversions in prior WDM transport schemes that transmit data serially in individual WDM optical channels.
Crosstalk and signal to noise degradation contributed by fiber nonlinearities in a WDM system are reduced according to the principles of the invention by transporting data in a parallel format using a plurality of optical channels in a WDM signal as a parallel bus and by coding the parallel-formatted data so that bit patterns in the parallel-formatted information that would otherwise contribute to nonlinear impairments are changed.
More specifically, a WDM parallel bus architecture is described in detail in my co-pending U.S. application Ser. No. 09/237,122 (Kartalopoulos 11), which is herein incorporated by reference in its entirety. Briefly, a WDM transport scheme characterized as a xe2x80x9cwavelength busxe2x80x9d transports data in a parallel format using a group of optical channels. As one example of the parallel format, an n-bit wide byte is transmitted using n optical channels (i.e., using n wavelengths) so that each of the n optical channels carries one of the n bits of the byte. As described in this co-pending application, the wavelength bus offers a more efficient use of bandwidth for transporting information in a WDM system than prior transport schemes.
In conjunction with the wavelength bus architecture, a coding scheme is employed to reduce the occurrence of bit patterns, e.g., xe2x80x9call onesxe2x80x9d, in the n-bit wide bytes that would give rise to four wave mixing or other nonlinear effects. In one exemplary embodiment for example, prior to converting data from electrical to optical format, bytes having xe2x80x9call onesxe2x80x9d or other defined bit patterns are detected and converted to xe2x80x9cnon-all onesxe2x80x9d bit patterns.
As compared to per-channel serial WDM transmission schemes, a wavelength bus architecture offers many advantages such as more efficient bandwidth utilization, reduced latency because of less serial-to-parallel and parallel-to-serial conversions, and sharing of resources among a group of wavelength channels, to name a few. By coding bytes being transmitted in parallel to eliminate or otherwise reduce the occurrence of xe2x80x9call onesxe2x80x9d or other bit conditions, the output power of a four-wave mixing product, for example, is minimized. By reducing the effects of nonlinear optical impairments in a WDM system operated according to the principles of the invention, several benefits can be realized. For example, channel density (i.e., number of optical channels/wavelengths) can be increased, thus increasing the aggregate bandwidth in a fiber. Moreover, power per wavelength can be increased, thus increasing the length of fiber spans without additional amplification.